I've been thinking about studying in this area for a long time now.
I have wanted to pursue these kind of studies for many years now.
ーI've been thinking about studying in this area for a long time now.
「長い間この分野の勉強をしようと考えていた」
I've been thinking about ...ing for a long time now. で「長い間…しようと考えていた」と表現できます。
ーI have wanted to pursue these kind of studies for many years now.
「何年もこれらに関する勉強を追求したいと思っていた」
I have wanted to pursue ... for many years now. で「何年も…を遂行・追求したいと思っていた」と表現できます。
ご参考まで！
カナダ 
